Nuclear magnetic relaxation differences provide contrast in magnetic images. The contrast may be controlled by control of magnetic spin relaxation with suitable agents; however, current attempts to use various agents have not had the advantage of a systematic study of those factors which provide maximal magnetic relaxation efficiency. This proposal focuses on the application of the magnetic field dependence of the nuclear magnetic relaxation rates in paramagnetic systems to develop a more complete understanding of the chemical and physical factors that control water proton relaxation. Experiments are proposed to examine the factors that control the electron spin relaxation rates that enter the problem by a systematic variation of the first coordination sphere ligand atom composition. Where appropriate the proton chemical exchange rates will be measured by oxygen-17 NMR methods. A quantitative assessment will be made of the outer sphere contributions to relaxation in both low molecular weight systems and in protein systems, and cage molecules will be examined. The thrust of the proposal is to attempt to characterize those chemical and physical factors that will lead to the best way to organize an optimally efficient relaxation agent. This work is preliminary to animal experiments and certainly preliminary to any direct clinical application.